The present invention relates to a rotor for an electrical machine and to an electrical machine including such a rotor.
It is known to generate electricity by using an engine to drive a generator. However, in some installations, such as aircraft, it is desirable to ensure that electricity can still be generated whilst the aircraft is in flight, even if all of the engines have failed. The electrical power may then be used to power the essential electrical/hydraulic systems and initiate an engine restart sequence.
Traditionally, emergency electrical power for an aircraft following engine failure is provided by a ram air turbine, RAT, which comprises an electrical generator equipped with a propeller. Under normal operating conditions the RAT is located within the aircraft fuselage and provides no electrical output. Following an engine failure, the pilot may choose to deploy the RAT. During deployment the RAT is moved by a support arm so as to place it in the airstream. Thus the propeller is caused to rotate due to the airflow caused by the forward movement of the aircraft. However, RATs are relatively heavy items which, by their very nature, are difficult to test.
It is known that, during flame out conditions, the low pressure, LP, shaft of a gas turbine aircraft engine continues to rotate due to the airflow past the by-pass fan blades. GB 2216603 discloses an arrangement in which an emergency generator can be coupled to the low pressure shaft, also known as low speed spool, of an aircraft gas turbine engine via a coupling unit. A sensor is provided to detect when propulsive power is lost from the engine, and thereby to connect the emergency power generator to the low speed spool during the flame out conditions. EP 0798454 describes a multi spool engine in which each of the spools is independently and directly coupled to an electrical generator. This document also discloses that one of the generators may act as an emergency power source in the event of engine failure due to the windmilling of the low speed spool.
The windmill speed of the engine low pressure shaft at minimum flight speed is approximately 200 to 250 rpm. This effectively defines the minimum speed at which each generator on the low pressure shaft is required to produce a reasonable electrical output. In the context of current and proposed aircraft systems, it is to be expected that each generator will produce something in the region of 25 kW under these conditions.
It is essential that the generator is reliable and is also able to withstand the harsh conditions encountered in a gas turbine engine environment. It is therefore desirable that there be no rotating coils or the use of slip rings within the generator. A switched reluctance generator provides a device in which their are no electrical connections to the rotor. However, in order to produce the required output of approximately 25 kW at a rotational speed of 250 rpm, it is estimated that the generator would weight several hundred kg. This size and weight penalty is unacceptable within a gas turbine engine environment. The weight of the generator can be reduced if the rotational speed is increased, and if the generator is driven through a step up gear box, for example with a step up ratio of 12:1 such that the minimum generator speed is 3000 rpm, then the required 25 kW output can be obtained with a much smaller and lighter generator weighing around 20-40 kg. However, an aircraft gas turbine engine at full speed typically has a low pressure shaft speed in excess of 3000 rpm and consequently the generator speed with a 12:1 step up gear box would be in excess of 36,000 rpm. It is possible to provide a clutch arrangement to isolate the drive to the generator at higher engine speeds, although this provides a further component which may fail thereby possibly depriving the aircraft from electrical power under emergency conditions. It is therefore necessary, for reasons of reliability, to avoid the use of a clutch, thereby providing a generator which is continuously connected to the low pressure shaft and which, therefore, must be able to withstand continuous rotation at high speed without suffering mechanical damage.
According to a first aspect of the present invention there is provided a rotor for a switched reluctance electrical machine, comprising a shaft carrying a plurality of laminations having a plurality of regions of a first radius interposed between regions of a second radius, wherein the first radius is greater than the second radius; and in which some of the laminations are provided with extensions in some or all of the second regions, the extensions having passages formed therein, and the laminations being held together by attachment elements which pass through the passages in the extensions.
It is thus possible to provide a rotor in which laminations, and especially thin laminations, can be held together by compression whilst providing support for the attachment elements so as to protect them against damage due to centrifugal forces.
It is known that rotating conductive elements within a magnetic circuit will experience eddy currents therein. Eddy current losses are reduced by using a laminated structure for the rotor. In order to reduce eddy current losses, and thereby to improve efficiency, the laminae, also known as laminations, should be thin. Preferably the laminations are less than 0.3 mm thick. In a preferred embodiment of the present invention, the rotor carries 600 individual laminations of 50% cobalt iron material (Rotalloy 3), each lamination being 0.2 mm thick with an oxide deposit for insulation purposes. Whilst the use of such thin laminations reduces eddy currents within the machine, which is particularly important given the high rotational rates which may be experienced by the machine, the use of a laminated rotor gives rise to problems in the operation of the machine at high speeds. At high rates of rotation, the forces on the laminated rotor will cause it to deform axially towards the centre of its supported length, potentially intruding into the rotor air gap and contacting the stator leading to catastrophic failure of the machine.
The rigidity of the lamination pack (and hence the rotor) may be improved by externally welding the laminations together. However, external welding of the lamination pack would lead to electrical shorting of the laminations and thus high eddy current losses. Bonding of the laminations is also not preferred in that the strength of available bonding materials is insufficient to withstand the forces imposed by high speeds of rotation. In addition where the laminations are glued the thickness of glue would be significant compared to the thickness of the laminations, and therefore would result in less laminations being used if the physical size of the electrical machine remains constrained, or would require the use of a physically larger machine in order to achieve similar electrical performance. Given these problems, the preferred method for forming a sufficiently rigid rotor is to hold the laminations together under mechanical compression, using retaining members at an increased radius.
Preferably the laminations are held between first and second flanges. The flanges are secured, for example by welding, to the rotor shaft. Additionally, one of the flanges may be integrally formed with the rotor shaft. Preferably a plurality of bolts extend between the flanges, thereby enabling the lamination assembly to be held in a compressed state.
The rotor of a variable reluctance machine comprises a plurality of radially spaced projections known as poles. The maximum power handling of such a machine, be it as a motor or a generator, is dependant on the magnitude of the change in magnetic properties between the projections and the gaps between the projections. It is therefore beneficial to ensure that the bolts securing the stack of laminations together do not pass through the body of the projections since this reduces the amount of magnetic material within the projections, thereby significantly reducing the performance of the electrical machine. It is therefore preferable to run substantially non-magnetic bolts in the gaps between the projections. For fast electrical machines, the centrifugal forces acting on the bolts may result in considerable bending stresses occurring within the bolts. This may lead to failure of the bolts, resulting in destruction of the generator. Even if the bolts do not fail completely, there is a concern that they may distort to such a degree that they contact a stator assembly, again resulting in failure of the electrical machine. Increasing the thickness of the bolts is not a preferred solution since larger diameter bolts are more difficult to accommodate within the available space between the poles of the rotor and they also result in an increase in the rotating mass which in turn increases the stress on the bolts. The applicant has realised that the laminations themselves can provide support for the bolts at at least one position along the length of the rotor.
Preferably the laminations provided with the extensions are arranged in groups, such that a plurality of laminations with the extensions co-operate to form a support region which holds the attachment elements against distortion due to centrifugal force. Advantageously the groups are spaced apart in such a manner so as to provide support for the bolts at regular intervals along their length. Thus, for example, in a rotor only having one group of laminations with the extensions, the group of laminations is provided at substantially the halfway point along the lamination stack. For a rotor having two groups of laminations providing support, these are positioned at substantially ⅓ and ⅔ along the length of the lamination stack.
Preferably the bolts are insulated from the laminations in order to avoid eddy current losses. Advantageously insulating bushes are provided. The bushes prevent the bolts from touching the laminations. Similarly, insulation is provided between the bolts and the flanges, or at least between the bolts and one of the flanges in order that a complete electrical circuit is not made. It is thus possible to provide a rotor suitable for use at high rotational rates.
According to a second aspect of the present invention, there is provided an electrical machine comprising a rotor according to the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a lamination having a plurality of projections thereon which form pole teeth, the lamination further comprising fixing support elements formed in a plurality of the inter-tooth regions.